Color and infrared image sensor

ABSTRACT

A color and infrared image sensor includes a silicon substrate, MOS transistors formed in the substrate and on the substrate, first photodiodes at least partly formed in the substrate, a photosensitive layer covering the substrate, and color filters, the photosensitive layer being interposed between the substrate and the color filters. The image sensor further includes first and second electrodes on either side of the photosensitive layer and delimiting second photodiodes in the photosensitive layer, the first photodiodes being configured to absorb the electromagnetic waves of the visible spectrum and of a first portion of the infrared spectrum and the photosensitive layer being configured to absorb the electromagnetic waves of the visible spectrum and to give way to the electromagnetic waves of said first portion of the infrared spectrum.

CROSS REFERENCE TO RELATED APPLICATIONS

The present patent application claims the priority benefit of Frenchpatent application FR19/02157 which is herein incorporated by reference.

FIELD

The present disclosure relates to an image sensor or electronic imager.

BACKGROUND

Image sensors are used in many fields, in particular in electronicdevices, due to their miniaturization. Image sensors are present, be itin man-machine interface applications or in image capture applications.

For certain applications, it is desirable to have an image sensorenabling to simultaneously acquire a color image and an infrared image.Such an image sensor is called color and infrared image sensor in thefollowing description. An example of application of a color and infraredimage sensor concerns the acquisition of an infrared image of an objecthaving a structured infrared pattern projected thereon. The fields ofuse of such image sensors particularly are motors vehicles, drones,smart phones, robotics, and augmented reality systems.

The phase during which a pixel collects charges under the action of anincident radiation is called integration phase of the pixel. Theintegration phase is generally followed by a readout phase during whichthe quantity of charges collected by the pixels is measured.

A plurality of constraints are to be taken into account for the designof a color and infrared image sensor. First, the resolution of the colorimages should not be smaller than that obtained with a conventionalcolor image sensor.

Second, for certain applications, it may be desirable for the imagesensor to be of global shutter type, that is, implementing an imageacquisition method where the beginnings and ends of pixel integrationphases are simultaneous. This may in particular apply for theacquisition of an infrared image of an object having a structuredinfrared pattern projected thereon.

Third, it is desirable for the size of the image sensor pixels to be assmall as possible. Fourth, it is desirable for the filling factor ofeach pixel, which corresponds to the ratio of the surface area, in topview, of the area of the pixel actively taking part in the capture ofthe incident radiation, to the total surface area, in top view, of thepixel, to be as large as possible.

It may be difficult to design a color and infrared image sensor whichfulfils all the previously-described constraints.

SUMMARY

An embodiment overcomes all or part of the disadvantages of thepreviously-described color and infrared image sensors.

According to an embodiment, the resolution of the color images acquiredby the color and infrared image sensor is greater than 2,560 ppi,preferably greater than 8,530 ppi.

According to an embodiment, the method of acquisition of an infraredimage is of global shutter type.

According to an embodiment, the size of the color and infrared imagesensor pixel is smaller than 10 μm, preferably smaller than 3 μm.

According to an embodiment, the filling factor of each pixel of thecolor and infrared image sensor is greater than 50%, preferably greaterthan 80%.

An embodiment provides a color and infrared image sensor comprising asilicon substrate, MOS transistors formed in the substrate and on thesubstrate, first photodiodes at least partly formed in the substrate, aphotosensitive layer covering the substrate, and color filters, thephotosensitive layer being interposed between the substrate and thecolor filters. The image sensor further comprises first and secondelectrodes on either side of the photosensitive layer and delimitingsecond photodiodes in the photosensitive layer, the first photodiodesbeing configured to absorb the electromagnetic waves of the visiblespectrum and of a first portion of the infrared spectrum and thephotosensitive layer being configured to absorb the electromagneticwaves of the visible spectrum and to give way to the electromagneticwaves of said first portion of the infrared spectrum.

According to an embodiment, the image sensor further comprises aninfrared filter, the color filters being interposed between thephotosensitive layer and the infrared filter, the infrared filter beingconfigured to give way to the electromagnetic waves of the visiblespectrum, to give way to the electromagnetic waves of said first portionof the infrared spectrum, and to block the electromagnetic waves of atleast a second portion of the infrared spectrum between the visiblespectrum and the first portion of the infrared spectrum.

According to an embodiment, the image sensor further comprises an arrayof lenses interposed between the photosensitive layer and the infraredfilter.

According to an embodiment, the image sensor further comprises, for eachpixel of the color image to be acquired, at least first, second, andthird sub-pixels, each comprising one of the second photodiodes, one ofthe first photodiodes, and one of the color filters, the color filtersof the first, second, and third sub-pixels giving way to electromagneticwaves in different frequency ranges of the visible spectrum.

According to an embodiment, for each pixel of the color image to beacquired, the second electrode is common to the first, second, and thirdsub-pixels.

According to an embodiment, the image sensor further comprises, for eachpixel of the color image to be acquired, at least one fourth sub-pixelcomprising one of the second photodiodes and one of the color filters,the color filter of the fourth sub-pixel being configured to block theelectromagnetic waves of the visible spectrum and to give way toelectromagnetic waves in a third portion of the infrared spectrumbetween the visible spectrum and the first portion of the infraredspectrum, the photosensitive layer being configured to absorbelectromagnetic waves in said third portion of the infrared spectrum.

According to an embodiment, the image sensor further comprises for eachfirst, second, and third sub-pixel a readout circuit coupled to thesecond photodiode and to the first photodiode.

According to an embodiment, the readout circuit is configured totransfer first electric charges generated in the first photodiode to afirst electrically-conductive track and configured to transfer secondcharges generated in the second photodiode to the firstelectrically-conductive track or a second electrically-conductive track.

According to an embodiment, the first photodiodes are arranged in rowsand in columns and the readout circuits are configured to control thegeneration of the first charges during first time intervals simultaneousfor all the first photodiodes of the image sensor.

According to an embodiment, the second photodiodes are arranged in rowsand in columns and the readout circuits are configured to control thegeneration of the second charges during second time intervalssimultaneous for all the second photodiodes of the image sensor orshifted in time from one row of second photodiodes to the other.

According to an embodiment, the readout circuits are configured tocontrol a first integration phase for the first photodiodes having afirst duration and to control a second integration phase for the secondphotodiodes having a second duration different from the first duration.

According to an embodiment, each readout circuit comprises at least onefirst follower-assembled MOS transistor, the second photodiode having afirst electrode directly coupled to the gate of the first MOS transistorand its second photodiode having a second electrode coupled to the gateof the first MOS transistor or to the gate of a secondfollower-assembled MOS transistor, via a third MOS transistor.

According to an embodiment, the photosensitive layer is made of organicmaterials and/or contains quantum dots.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features and advantages, as well as others, will bedescribed in detail in the following description of specific embodimentsgiven by way of illustration and not limitation with reference to theaccompanying drawings, in which:

FIG. 1 is a partial simplified exploded perspective view of anembodiment of a color and infrared image sensor;

FIG. 2 is a partial simplified cross-section view of the image sensor ofFIG. 1 illustrating an embodiment of the electrodes of the image sensor;

FIG. 3 is a partial simplified cross-section view of the image sensor ofFIG. 1 illustrating another embodiment of the electrodes;

FIG. 4 is an electric diagram of an embodiment of a readout circuit of asub-pixel of the image sensor of FIG. 1;

FIG. 5 is an electric diagram of another embodiment of the readoutcircuit;

FIG. 6 is an electric diagram of another embodiment of the readoutcircuit;

FIG. 7 is an electric diagram of another embodiment of the readoutcircuit;

FIG. 8 is an electric diagram of another embodiment of the readoutcircuit;

FIG. 9 is an electric diagram of another embodiment of the readoutcircuit;

FIG. 10 is a timing diagram of signals of an embodiment of an operatingmethod of the image sensor having the readout circuit of FIG. 4;

FIG. 11 is a timing diagram of signals of another embodiment of anoperating method of the image sensor having the readout circuit of FIG.4;

FIG. 12 is a timing diagram of signals of an embodiment of an operatingmethod of the image sensor having the readout circuit of FIG. 9; and

FIG. 13 is a timing diagram of signals of an embodiment of an operatingmethod of the image sensor having the readout circuit of FIG. 9.

DESCRIPTION OF THE EMBODIMENTS

Like features have been designated by like references in the variousfigures. In particular, the structural and/or functional features thatare common among the various embodiments may have the same referencesand may dispose identical structural, dimensional and materialproperties. For clarity, only those steps and elements which are usefulto the understanding of the described embodiments have been shown andare detailed. In particular, what use is made of the image sensorsdescribed hereafter has not been detailed.

In the following disclosure, unless indicated otherwise, when referenceis made to absolute positional qualifiers, such as the terms “front”,“back”, “top”, “bottom”, “left”, “right”, etc., or to relativepositional qualifiers, such as the terms “above”, “below”, “higher”,“lower”, etc., or to qualifiers of orientation, such as “horizontal”,“vertical”, etc., reference is made to the orientation shown in thefigures, or to an image sensor as orientated during normal use. Unlessspecified otherwise, the expressions “around”, “approximately”,“substantially” and “in the order of” signify within 10%, and preferablywithin 5%.

Unless indicated otherwise, when reference is made to two elementsconnected together, this signifies a direct connection without anyintermediate elements other than conductors, and when reference is madeto two elements coupled together, this signifies that these two elementscan be connected or they can be coupled via one or more other elements.Further, a signal which alternates between a first constant state, forexample, a low state, noted “0”, and a second constant state, forexample, a high state, noted “1”, is called “binary signal”. The highand low states of different binary signals of a same electronic circuitmay be different. In particular, the binary signals may correspond tovoltages or to currents which may not be perfectly constant in the highor low state. Further, it is here considered that the terms “insulating”and “conductive” respectively mean “electrically insulating” and“electrically conductive”.

The transmittance of a layer corresponds to the ratio of the intensityof the radiation coming out of the layer to the intensity of theradiation entering the layer. In the following description, a layer or afilm is called opaque to a radiation when the transmittance of theradiation through the layer or the film is smaller than 10%. In thefollowing description, a layer or a film is called transparent to aradiation when the transmittance of the radiation through the layer orthe film is greater than 10%. In the following description, therefraction index of a material corresponds to the refraction index ofthe material for the wavelength range of the radiation captured by theimage sensor. Unless specified otherwise, the refraction index isconsidered as substantially constant over the wavelength range of theuseful radiation, for example, equal to the average of the refractionindex over the wavelength range of the radiation captured by the imagesensor.

In the following description, “visible light” designates anelectromagnetic radiation having a wavelength in the range from 400 nmto 700 nm and “infrared radiation” designates an electromagneticradiation having a wavelength in the range from 700 nm to 1 mm. Ininfrared radiation, one can particularly distinguish near infraredradiation having a wavelength in the range from 700 nm to 1.4 μm.

A pixel of an image corresponds to the unit element of the imagecaptured by an image sensor. When the optoelectronic device is a colorimage sensor, it generally comprises, for each pixel of the color imageto be acquired, at least three components which each acquire a lightradiation substantially in a single color, that is, in a wavelengthrange below 100 nm (for example, red, green, and blue). Each componentmay particularly comprise at least one photodetector.

FIG. 1 is a partial simplified exploded perspective view and FIG. 2 is apartial simplified cross-section view of an embodiment of a color andinfrared image sensor 1. Image sensor 1 comprises an array of firstphoton sensors 2, also called photodetectors, capable of capturing aninfrared image, and an array of second photodetectors 4, capable ofcapturing a color image. The arrays of photodetectors 2 and 4 areassociated with an array of readout circuits 6 measuring the signalscaptured by photodetectors 2 and 4. Readout circuit means an assembly oftransistors for reading out, addressing, and controlling the pixel orsub-pixel defined by the corresponding photodetectors 2 and 4.

In image sensor 1, the array of color photodetectors 4 covers the arrayof infrared photodetectors 2. For each pixel of the color image to beacquired, call sub-pixel SPix of image sensor 1 the portion of imagesensor 1 comprising photodetector 4 enabling to acquire the lightradiation in a limited portion of the visible radiation of the image tobe acquired.

Four sub-pixels SPix have been shown in FIGS. 1 and 2. For clarity, onlycertain elements of image sensor 1 present in FIG. 2 are shown inFIG. 1. Image sensor 1 comprises, from bottom to top in FIG. 2:

a semiconductor substrate 10 comprising an upper surface 12, preferablyplanar;for each sub-pixel SPix, at least one doped semiconductor region 14formed in substrate 12 and forming part of infrared photodiode 2;electronic components 16 of readout circuits 6 located in substrate 10and/or on surface 12, a single component 16 being shown per sub-pixelSPix in FIG. 2;a stack 18 of insulating layers covering surface 12, conductive tracks20 being located on stack 18 and between the insulating layers of stack18;for each sub-pixel SPix, an electrode 22 resting on stack 18 and coupledto substrate 10, to one of components 16, or to one of the conductivetracks 20 by a conductive via 24;an active layer 26 covering all the electrodes 22 and covering stack 18between electrodes 22;for each sub-pixel SPix, an electrode 28 resting on active layer 26 andcoupled to substrate 10, to one of components 16, or to one ofconductive tracks 20 by a conductive via 30;an insulating layer 32 covering all the electrodes 28 and covering theactive layer 26 between electrodes 28;for each sub-pixel SPix, a color filter 34 covering insulating layer 32;for each sub-pixel SPix, a microlens 36 covering color filter 34;an insulating layer 38 covering microlenses 36; anda filter 40 covering insulating layer 36.

Sub-pixels SPix may be distributed in rows and in columns. In thepresent embodiment, each sub-pixel SPix has, in a directionperpendicular to surface 12, a square or rectangular base with a sidelength varying from 0.1 μm to 100 μm, for example, equal toapproximately 3 μm. However, each sub-pixel SPix may have a base with adifferent shape, for example, hexagonal.

In the present embodiment, active layer 26 is common to all thesub-pixels SPix of image sensor 1. The active area of each colorphotodetector 4 corresponds to the area where most of the incidentradiation is absorbed and converted into an electric signal by colorphotodetector 4 and substantially corresponds to the portion of theactive layer 26 located between lower electrode 22 and upper electrode28.

According to an embodiment, each infrared photodetector 2 is capable ofcapturing an electromagnetic radiation in a wavelength range in therange from 400 nm to 1,100 nm. According to an embodiment, active layer26 is capable of capturing an electromagnetic radiation in a wavelengthrange in the range from 400 nm to 700 nm, that is, of only absorbingvisible light. According to another embodiment, active layer 26 iscapable of capturing an electromagnetic radiation in a wavelength rangein the range from 400 nm to 920 nm, that is, visible light and a portionof near infrared. The photodetectors may be made of organic materials.The photodetectors may correspond to organic photodiodes (OPD) or toorganic photoresistors. In the following description, it is consideredthat the photodetectors correspond to photodiodes.

Filter 40 is capable of giving way to visible light, of giving way tothe portion of the infrared radiation over the infrared wavelength rangeof interest for the acquisition of the infrared image, and of blockingthe rest of the incident radiation, and particularly the rest of theinfrared radiation outside of the infrared wavelength range of interest.According to an embodiment, the infrared wavelength range of interestmay correspond to a 50 nm range centered on the expected wavelength ofthe infrared radiation, for example, centered on the 940 nm wavelengthor centered on the 850 nm wavelength. Filter 40 may be an interferencefilter and/or comprise absorbing and/or reflective layers.

Color filters 34 may correspond to colored resin blocks. Each colorfilter 34 is capable of giving way to the infrared radiation, forexample, at a wavelength between 700 nm and 1 mm and, for at least someof the color filters, of giving way to a wavelength range of visiblelight. For each pixel of the color image to be acquired, the imagesensor may comprise a sub-pixel SPix having its color filter 34 onlycapable of giving way to blue light, for example, in the wavelengthrange from 430 nm to 490 nm, a sub-pixel SPix having its color filter 34only capable of giving way to green light, for example, in thewavelength range from 510 nm to 570 nm, and a sub-pixel SPix having itscolor filter 34 only capable of giving way to red light, for example, inthe wavelength range from 600 nm to 720 nm.

According to an embodiment, active layer 26 is capable of capturing anelectromagnetic radiation in a wavelength range in the range from 400 nmto 700 nm, that is, of only absorbing visible light. Color filters 34may then be distributed in a Bayer array. Thereby, for each sub-pixelSPix, the color photodetector 4 of the sub-pixel only captures theportion of the visible light having crossed the color filter 34 of thesub-pixel.

According to another embodiment, active layer 26 is capable of capturingan electromagnetic radiation in a wavelength range in the range from 400nm to 920 nm, that is, visible light and a portion of near infrared. Inthis case, one of color filters 34 is capable of only giving way toinfrared radiation and of blocking visible light. One of the colorphotodiodes 4 then plays the role of a photodiode for near infrared.This may be advantageous for the acquisition of color images,particularly in case of a low luminosity. The infrared radiationcaptured by infrared photodiodes 2 corresponds to a wavelength rangedifferent from the infrared radiation captured by color photodiode 4playing the role of a photodiode for near infrared.

According to an embodiment, semiconductor substrate is made of silicon,preferably, of single crystal silicon. The substrate may be ofsilicon-on-insulator or SOI type comprising a stack of a silicon layeron an insulating layer. According to an embodiment, electroniccomponents 16 comprise transistors, particularly metal-oxide gatefield-effect transistors, also called MOS transistors. Infraredphotodiodes 2 are inorganic photodiodes, preferably made of silicon.Each infrared photodiode 2 comprises at least the doped silicon region14 which extends in substrate 10 from surface 12. According to anembodiment, substrate 10 is non-doped or lightly doped with a firstconductivity type, for example, type P, and each region 14 is a dopedregion, of the conductivity type opposite to that of substrate 10, forexample, type N. The depth of each region 14, measured from surface 12,may be in the range from 1 μm to 12 μm. Infrared photodiode 2 maycorrespond to a pinned photodiode. Examples of pinned photodiodes areparticularly described in U.S. Pat. No. 6,677,656. These for example arephotodiodes separated by deep insulating trenches to ease the collectionof the charges generated in depth due to the near infrared radiation.

Conductive tracks 20, conductive vias 24, 30 may be made of a metallicmaterial, for example, silver (Ag), aluminum (Al), gold (Au), copper(Cu), nickel (Ni), titanium (Ti), and chromium (Cr). Conductive tracks20 and conductive vias 24, 30 may have a monolayer or multilayerstructure. Each insulating layer of stack 18 may be made of an inorganicmaterial, for example, made of silicon oxide (SiO₂) or a silicon nitride(SiN).

Each electrode 22, 28 is at least partially transparent to the lightradiation that it receives. Each electrode 22, 28 may be made of atransparent conductive material, for example, of transparent conductiveoxide or TCO, of carbon nanotubes, of graphene, of a conductive polymer,of a metal, or of a mixture or an alloy of at least two of thesecompounds. Each electrode 22, 28 may have a monolayer or multilayerstructure.

Examples of TCOs capable of forming each electrode 22, 28 are indium tinoxide (ITO), aluminum zinc oxide (AZO), and gallium zinc oxide (GZO),titanium nitride (TiN), molybdenum oxide (MoO₃), and tungsten oxide(WO₃). An example of a conductive polymer capable of forming eachelectrode 22, 28 is the polymer known as PEDOT:PSS, which is a mixtureof poly(3,4)-ethylenedioxythiophene and of sodium poly(styrenesulfonate), and polyaniline, also called PAni. Examples of metalscapable of forming each electrode 22, 28 are silver, aluminum, gold,copper, nickel, titanium, and chromium. An example of a multilayerstructure capable of forming each electrode 22, 28 is a multilayer AZOand silver structure of AZO/Ag/AZO type.

The thickness of each electrode 22, 28 may be in the range from 10 nm to5 μm, for example, in the order of 30 nm. In the case where electrode22, 28 is metallic, the thickness of electrode 22, 28 is smaller than orequal to 20 nm, preferably smaller than or equal to 10 nm.

Each insulating layer 32, 38 may be made of a fluorinated polymer,particularly the fluorinated polymer commercialized under trade nameCytop by Bellex, of polyvinylpyrrolidone (PVP), of polymethylmethacrylate (PMMA), of polystyrene (PS), of parylene, of polyimide(PI), of acrylonitrile butadiene styrene (ABS), of poly(ethyleneterephtalate) (PET), of poly(ethylene napthalate) (PEN), of cyclo olefinpolymer (COP), en polydimethylsiloxane (PDMS), of a photolithographyresin, of epoxy resin, of acrylate resin, or of a mixture of at leasttwo of these compounds. As a variation, each insulating layer 32, 38 maybe made of an inorganic dielectric, particularly made of siliconnitride, of silicon oxide, or of aluminum oxide (Al₂O₃). The aluminumoxide may be deposited by atomic layer deposition (ALD). The maximumthickness of each insulating layer 32, 38 may be in the range from 50 nmto 2 μm, for example, in the order of 100 nm.

Active layer 26 may comprise small molecules, oligomers, or polymers.These may be organic or inorganic materials, particularly quantum dots.Active layer 26 may comprise an ambipolar semiconductor material, or amixture of an N-type semiconductor material and of a P-typesemiconductor material, for example in the form of stacked layers or ofan intimate mixture at a nanometer scale to form a bulk heterojunction.The thickness of active layer 26 may be in the range from 50 nm to 2 μm,for example, in the order of 200 nm.

Example of P-type semiconductor polymers capable of forming active layer26 are poly(3-hexylthiophene) (P3HT),poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2′,1′,3′-benzothiadiazole)](PCDTBT), poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene))-2,6-diyl] (PBDTTT-C),poly[2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene-vinylene] (MEH-PPV),orpoly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)](PCPDTBT).

Examples of N-type semiconductor materials capable of forming activelayer 26 are fullerenes, particularly C60, [6,6]-phenyl-C₆₁-methylbutanoate ([60]PCBM), [6,6]-phenyl-C₇₁-methyl butanoate ([70]PCBM),perylene diimide, zinc oxide (ZnO), or nanocrystals enabling to formquantum dots.

Active layer 26 may be interposed between first and second interfacelayers, not shown. According to the photodiode polarization mode, theinterface layers ease the collection, the injection, or the blocking ofcharges from the electrodes into active layer 26. The thickness of eachinterface layer is preferably in the range from 0.1 nm to 1 μm. Thefirst interface layer enables to align the work function of the adjacentelectrode with the electronic affinity of the acceptor material used inactive layer 26. The first interface layer may be made of cesiumcarbonate (CSCO₃), of metal oxide, particularly of zinc oxide (ZnO), orof a mixture of at least two of these compounds. The first interfacelayer may comprise a self-assembled monomolecular layer or a polymer,for example, (polyethyleneimine, ethoxylated polyethyleneimine,poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)].The second interface layer enables to align the work function of theother electrode with the ionizing potential of the donor material usedin active layer 26. The second interface layer may be made of copperoxide (CuO), of nickel oxide (NiO), of vanadium oxide (V₂O₅), ofmagnesium oxide (MgO), of tungsten oxide (WO₃), of molybdenum oxide(MoO₃), of PEDOT:PSS, or of a mixture of at least two of thesecompounds.

Microlenses 36 have a micrometer-range size. In the present embodiment,each sub-pixel SPix comprises a microlens 36. As a variation, eachmicrolens 36 may be replaced with another type of micrometer-rangeoptical element, particularly a micrometer-range Fresnel lens, amicrometer-range index gradient lens, or a micrometer-range diffractiongrating. Microlenses 36 are converging lenses each having a focaldistance f in the range from 1 μm to 100 μm, preferably from 1 μm to 10μm. According to an embodiment, all the microlenses 36 are substantiallyidentical.

Microlenses 36 may be made of silica, of PMMA, of a positivephotosensitive resin, of PET, of PEN, of COP, of PDMS/silicone, or ofepoxy resin. Microlenses 36 may be formed by flowing of resist blocks.Microlenses 36 may further be formed by molding on a layer of PET, PEN,COP, PDMS/silicone or epoxy resin.

According to an embodiment, layer 38 is a layer which follows the shapeof microlenses 36. Layer 38 may be obtained from an optically clearadhesive (OCA), particularly a liquid optically clear adhesive (LOCA),or a material with a low refraction index, or an epoxy/acrylate glue, ora film of a gas or of a gaseous mixture, for example, air. Preferably,when layer 38 follows the shape of microlenses 36, layer 38 is made of amaterial having a low refraction index, lower than that of the materialof microlenses 36. Layer 38 may be made of a filling material which is anon-adhesive transparent material. According to another embodiment,layer 38 corresponds to a film which is applied against microlens array36, for example, an OCA film. In this case, the contact area betweenlayer 38 and microlenses 36 may be decreased, for example, limited tothe top of the microlenses. Layer 38 may then be formed of a materialhaving a higher refraction index than in the case where layer 38 followsthe shape of microlenses 36. According to another embodiment, layer 38corresponds to an OCA film which is applied against microlens array 36,the adhesive having properties which enable film 38 to completely orsubstantially completely follow the surface of the microlenses.

According to the considered materials, the method of forming at leastcertain layers of image sensor 1 may correspond to a so-called additiveprocess, for example, by direct printing of the material forming theorganic layers at the desired locations, particularly in sol-gel form,for example, by inkjet printing, photogravure, silk-screening,flexography, spray coating, or drop casting. According to the consideredmaterials, the method of forming the layers of image sensor 1 maycorrespond to a so-called subtractive method, where the material formingthe organic layers is deposited all over the structure and where thenon-used portions are then removed, for example, by photolithography orlaser ablation. Further, according to the considered layers andmaterials, the material forming active layer 26 is deposited over theentire structure and is not partially removed, the pitch of thephotodiodes being then obtained by the position of electrodes 22 and 28.According to the considered material, the deposition over the entirestructure may be performed, for example, by liquid deposition, bycathode sputtering, or by evaporation. Methods such as spin coating,spray coating, heliography, slot-die coating, blade coating,flexography, or silk-screening, may in particular be used. When thelayers are metallic, the metal is for example deposited by evaporationor by cathode sputtering over the entire support and the metal layersare delimited by etching.

Advantageously, at least some of the layers of image sensor 1 may beformed by printing techniques. The materials of the previously-describedlayers may be deposited in liquid form, for example, in the form ofconductive and semiconductor inks by means of inkjet printers.“Materials in liquid form” here also designates gel materials capable ofbeing deposited by printing techniques. Anneal steps may be providedbetween the depositions of the different layers, but it is possible forthe anneal temperatures not to exceed 150° C., and the deposition andthe possible anneals may be carried out at the atmospheric pressure.

FIG. 3 is a cross-section view of image sensor 1 illustrating anotherembodiment of a layout of the electrodes of color photodiodes 4. In thepresent embodiment, electrode 28 is common to each color photodiode 4 ofthe pixel. Further, electrode 28 may be common to all the pixels of asame row of pixels. Via 30 may then not be present for each sub-pixel ofthe pixel and may be provided in areas which do not correspond tosub-pixels, for example, at the pixel periphery. Only electrode 22 isdelimited for each sub-pixel SPix.

FIG. 4 shows the simplified electric diagram of an embodiment of thereadout circuit 6-1 associated with the color photodiode 4 and with theinfrared photodiode 2 of a sub-pixel SPix.

Readout circuit 6-1 comprises a MOS transistor in a follower assembly60, in series with a selection MOS transistor 62, between two terminals64, 66. Terminal 64 is coupled to a source of a high reference potentialVDD in the case where the transistors forming the readout circuits areN-channel MOS transistors, or of a low reference potential, for example,the ground, in the case where the transistors forming the readoutcircuit are P-channel MOS transistors. Terminal 66 is coupled to aconductive track 68. Conductive track 68 may be coupled to all thesub-pixels of a same column and may be coupled to a current source 69which does not belong to sub-pixel readout circuit 6-1. The gate oftransistor 62 is intended to receive a sub-pixel selection signal SEL.The gate of transistor 60 is coupled to a node FD. Node FD is coupled,by a MOS reset transistor 70, to a terminal of application of a resetpotential Vrst, which potential may be VDD. The gate of transistor 70 isintended to receive a signal RST for controlling the resetting of thesub-pixel, particularly enabling to reset node FD substantially topotential Vrst.

Node FD is coupled to the cathode electrode 22 of the color photodiode 4of the considered sub-pixel via a MOS transfer transistor 72. The anodeelectrode 28 of the color photodiode 4 of the sub-pixel is coupled to asource of a reference potential V_RGB. The gate of transistor 72 isintended to receive a signal TG_RGB for selecting the color photodiode 4of the sub-pixel. The gate of transistor 60 is further coupled to thecathode electrode of the infrared photodiode 2 of the consideredsub-pixel via a MOS transfer transistor 74. The anode electrode of theinfrared photodiode 2 of the sub-pixel is coupled to a source of a lowreference potential GND, for example, the ground. The gate of transistor74 is intended to receive a signal TG_IR for selecting the infraredphotodiode 2 of the sub-pixel. In the present embodiment, the readoutcircuit 6-1 shown in FIG. 4 comprising five MOS transistors is providedfor each sub-pixel. For each row of sub-pixels, signals SEL, TR_RGB,TR_IR, RST and potential V_RGB may be transmitted to all the sub-pixelsin the row. Call V_FD the potential at node FD referenced to lowreference potential GND. A capacitor, not shown, having an electrodecoupled to node FD and having its other electrode coupled to the sourceof low reference potential GND may be provided. As a variation, the roleof this capacitor may be fulfilled by the stray capacitances present atnode FD.

FIG. 5 shows the simplified electric diagram of another embodiment ofthe readout circuit 6-2 associated with the color photodiode 4 and withthe infrared photodiode 2 of a sub-pixel. Readout circuit 6-2 comprisesall the elements of the readout circuit 6-1 shown in FIG. 4, with thedifference that transfer transistor 72 is not present, cathode electrode22 being directly coupled to the gate of follower-assembled transistor60.

FIG. 6 shows the simplified electric diagram of another embodiment ofthe readout circuit 6-3 associated with the color photodiode 4 and withthe infrared photodiode 2 of a sub-pixel. Readout circuit 6-3 comprisesall the elements of the readout circuit 6-1 shown in FIG. 4 and furthercomprises, for each conductive track 68, an operational amplifier 76having its inverting input (−) coupled to conductive track 68, havingits non-inverting input (+) coupled to a source of a reference potentialVref, and having its output delivering the potential Vrst applied to oneof the power terminals of reset transistor 70. Operational amplifier 76may be coupled to all the reset transistors 70 of the sub-pixels coupledto conductive track 68. Operational amplifier 76 forms a feedback loopwhich enables to decrease, or even to suppress, the thermal noise ofreset transistor 70, such noise being usually suppressed by a readoutmethod implementing a correlated double sampling (CDS).

FIG. 7 shows the simplified electric diagram of another embodiment ofthe readout circuit 6-4 associated with the color photodiode 4 and withthe infrared photodiode 2 of a sub-pixel. Readout circuit 6-4 comprisesall the elements of the readout circuit 6-1 shown in FIG. 4, with thedifference that MOS transfer transistor 72 is not present, that thecathode electrode 22 of color photodiode 4 is connected to the gate offollower-assembled transistor 60, and that readout circuit 6-4 furthercomprises a MOS transistor in a follower assembly 78, in series with aMOS selection transistor 80, between two terminals 82, 84. Terminal 82is coupled to the source of high reference potential VDD. Terminal 84 iscoupled to conductive track 68. The gate of transistor 80 is intended toreceive a signal SEL′ of selection of infrared photodiode 2. The gate oftransistor 78 is coupled, by a MOS reset transistor 86, to a terminal ofapplication of reset potential Vrst. The gate of transistor 86 isintended to receive a signal RST′ for controlling the resetting ofinfrared photodiode 2 enabling to recharge photodiode 2 by applying apotential Vrst to the cathode of the infrared photodiode. The gate oftransistor 78 is coupled to transfer transistor 74.

FIG. 8 shows the simplified electric diagram of another embodiment ofthe readout circuit 6-5 associated with the color photodiode 4 and withthe infrared photodiode 2 of a sub-pixel. Readout circuit 6-5 comprisesall the elements of the readout circuit 6-4 shown in FIG. 7 and furthercomprises the operational amplifier 76 of the readout circuit 6-3 shownin FIG. 6 having its inverting input (−) coupled to conductive track 68,having its non-inverting input (+) coupled to the source of a referencepotential Vref, and having its output delivering the potential Vrstapplied to one of the power terminals of reset transistors 70 and 86.

FIG. 9 shows the simplified electric diagram of another embodiment ofthe readout circuit 6-6 associated with the color photodiode 4 and withthe infrared photodiode 2 of a sub-pixel. Readout circuit 6-6 comprisesall the elements of the readout circuit 6-4 shown in FIG. 7, with thedifference that selection transistor 80 is coupled to a conductive track88, different from conductive track 68, and which is coupled to acurrent source 89. In the present embodiment, the color sub-pixels andthe infrared sub-pixels are thus not coupled in a column. This enablesto implement readout methods differing, in particular, by the durationsof the successive steps of the method, for color pixels and infraredpixels.

Generally, reset potentials Vrst, Vrst′ may be common to all pixels.Potential Vrst is then equal to potential Vrst′. As a variation, thereset potentials may be differentiated according to the column fromwhich the corresponding pixels are read.

FIG. 10 is a timing diagram of binary signals RST, TG_IR, TG_RGB, andSEL, and of potentials V_RGB, and V_FD during an embodiment of a methodof operation of the image sensor 6-1 shown in FIG. 4. Call t0 to t9successive times of an operating cycle. The timing diagram has beenestablished by considering that the MOS transistors of readout circuit6-1 are N-channel transistors.

At time t0, signal SEL is in the low state so that selection transistor62 is off. The cycle comprises a reset phase. For this purpose, signalRST is in the high state so that reset transistor 70 is conductive.Signal TG_IR is in the high state so that transfer transistor 74 isconductive. The charges accumulated in infrared photodiode 2 are thendischarged towards the source of potential Vrst. Similarly, signalTG_RGB is in the high state so that transfer transistor 72 isconductive. Color photodiode 4 is charged by injecting charges viapotential source Vrst.

Just before time t1, potential V_RGB is set to a low level. At time t1,which marks the beginning of a new cycle, signal TG_IR is set to the lowstate so that transfer transistor 74 is turned off and signal TG_RGB isset to the low state so that transfer transistor 72 is turned off. Anintegration phase then starts, during which charges are generated inphotodiodes 2 and 4. Just after time t1, signal RST is set to the lowstate so that reset transistor 70 is turned off. Potential V_FD is thenset to a first value V1. Between times t1 and t2, signal SEL istemporarily set to a high state, so that the potential of conductivetrack 68 reaches a value representative of V1, which is stored.

At time t2, potential V_RGB is set to a high level, which stops thecharge collection in color photodiode 4. Between times t2 and t3, signalTG_IR is set to the high state so that transfer transistor 74 isconductive. The charges stored in infrared photodiode 2 are thentransferred to node FD, having its potential V_FD decreasing to a valueV2. Between times t3 and t4, signal SEL is temporarily set to the highstate, so that the potential of conductive track 68 reaches a valuerepresentative of V2, which is stored. The difference between values V2and V1 is representative of the quantity of charges collected ininfrared photodiode 2 during the integration phase.

Between times t4 and t5, signal RST is set to the high state so thatreset transistor 70 is conductive. Potential V_FD then substantiallystabilizes at a value V3. Between times t5 and t6, signal SEL istemporarily set to the high state, so that the potential of conductivetrack 68 reaches a value representative of V3, which is stored. Betweentimes t6 and t7, signal TG_RGB is set to the high state so that transfertransistor 72 is conductive. The charges collected in color photodiode 4are then transferred to node FD, which has its potential V_FD decreasingto a value V4. Between times t7 and t8, signal SEL is temporarily set tothe high state, so that the potential of conductive track 68 reaches avalue representative of V4, which is stored. The difference betweenvalues V4 and V3 is representative of the quantity of charges stored incolor photodiode 4 during the integration phase. Time t9 marks the endof the cycle and corresponds to the time t1 of the next cycle. In thepresent embodiment, the duration of the integration phase of infraredphotodiode 2 is controlled by transfer transistor 74 while the durationof the integration phase of color photodiode 4 is controlled bypotential V_RGB. In the present embodiment, the integration phase ofcolor photodiode 4 has the same duration as the integration phase ofinfrared photodiode 2.

The present embodiment advantageously enables to carry out a readoutmethod of global shutter type for the acquisition of color images, wherethe integration phases of all the color photodiodes are simultaneouslycarried out, and a readout method of global shutter type for theacquisition of the infrared image, where the integration phases of allthe color photodiodes are simultaneously carried out.

FIG. 11 is a timing diagram of binary signals RST, TG_IR, TG_RGB, andSEL, and of potentials V_RGB, and V_FD during another embodiment of amethod of operation of the image sensor 6-1 shown in FIG. 5. As comparedwith the timing diagram shown in FIG. 10, the signal T_IR in FIG. 11varies like signal T_RGB in FIG. 10 and signal T_RGB is not present inFIG. 11. In the present embodiment, the duration of the integrationphase of infrared photodiode 2 is controlled by transfer transistor 74while the duration of the integration phase of color photodiode 4 iscontrolled by potential V_RGB. The reading is thus carried out in twosteps. A first readout step is carried out between times t3 and t4 wherethe value of color photodiode 4 is read out and a second readout step iscarried out between times t7 and t8, where the sum of the value of colorphotodiode 4 and of the value of infrared photodiode is read out. Thevalue of infrared photodiode 2 is obtained by an operation ofsubtraction of the two read values. In the present embodiment, theintegration phase of color photodiode 4 is shorter than the integrationphase of infrared photodiode 2. The present embodiment advantageouslyenables to carry out a global shutter type readout method for theacquisition of the color image, where the integration phases of all thecolor photodiodes are carried out simultaneously, and a global shuttertype readout method for the acquisition of the infrared image, where theintegration phases of all the infrared photodiodes are carried outsimultaneously.

FIG. 12 is a timing diagram of binary signals RST, TG_IR, TG_RGB, andSEL, and of potentials V_RGB and V_FD during another embodiment of amethod of operation of the image sensor 6-1 shown in FIG. 4. The presentoperating cycle comprises the same succession of phases as the operatingcycle illustrated in FIG. 10, with the difference that potential V_RGBremains in the low state until a time t1′, in the present embodimentbetween times t1 and t2.

In the present embodiment, for each sub-pixel, the integration phase ofinfrared photodiode 2 extends from time t1 to time t2 and theintegration phase of color photodiode 4 extends from time t1 to timet1′. The present embodiment enables the duration of the integrationphase of the infrared photodiode to be different from the duration ofthe color photodiode integration phase. Further, the present embodimentadvantageously enables to carry out a readout method of global shuttertype for the acquisition of the color image and a readout method ofglobal shutter type for the acquisition of the infrared image.

FIG. 13 is a timing diagram of signals RST′_1, RST′_2, RST_1, RST_2,SEL_1, SEL_2 and of potentials V_RGB 1 and V_RGB 2 during anotherembodiment of a method of operation of the image sensor 6-6 shown inFIG. 9, considering first and second successive pixel rows, the signalsand potentials associated with the first row comprising suffix “_1” andthe signals and potentials associated with the second row comprisingsuffix “_2”.

As shown in FIG. 13, signals V_RGB 1 and V_RGB 2 are permanentlymaintained in the low state. The integration phases of the infraredphotodiodes of the two rows controlled by signals TG_IR 1 and TG_IR 2are carried out simultaneously while the integration phase of the colorphotodiode of the first row controlled by signals RST_1 and SEL_1 isshifted in time with respect to the integration phase of the colorphotodiode of the second row controlled by signals RST_2 and SEL_2. Thisenables to implement a global shutter type readout method for theinfrared photodiodes and a rolling shutter type readout method for thecolor photodiodes during which the integration phases of the pixel rowsare shifted in time with respect to one another.

Various embodiments and variants have been described. It will beunderstood by those skilled the art that certain characteristic of thesevarious alterations, modifications, and improvements, and othervariations will occur to those skilled in the art. In particular, thereadout circuits shown in FIGS. 5 to 8 may be implemented with any ofthe sub-pixel structures shown in FIGS. 2 and 3. Further, the timingdiagrams previously described in relation with FIGS. 9 and 10 mayfurther be implemented with the readout circuits shown in FIGS. 5 to 8.Finally, the practical implementation of the embodiments and variantsdescribed herein is within the capabilities of those skilled in the artbased on the functional description provided hereinabove.

1. A color and infrared image sensor comprising a silicon substrate, MOStransistors formed in the substrate and on the substrate, firstphotodiodes at least partly formed in the substrate, a photosensitivelayer covering the substrate, and color filters, the photosensitivelayer being interposed between the substrate and the color filters, theimage sensor further comprising first and second electrodes on eitherside of the photosensitive layer and delimiting second photodiodes inthe photosensitive layer, the first photodiodes being configured toabsorb the electromagnetic waves of the visible spectrum and of a firstportion of the infrared spectrum and the photosensitive layer beingconfigured to absorb the electromagnetic waves of the visible spectrumand to give way to the electromagnetic waves of said first portion ofthe infrared spectrum, the image sensor comprising, for each pixel ofthe color image to be acquired, at least first, second, and thirdsub-pixels each comprising one of the second photodiodes, one of thefirst photodiodes, and one of the color filters, the color filters ofthe first, second, and third sub-pixels giving way to electromagneticwaves in different frequency ranges of the visible spectrum, the imagesensor comprising, for each pixel of the color image to be acquired, atleast one fourth sub-pixel comprising one of the second photodiodes andone of the color filters, the color filter of the fourth sub-pixel beingconfigured to block the electromagnetic waves of the visible spectrumand to give way to electromagnetic waves in a third portion of theinfrared spectrum between the visible spectrum and the first portion ofthe infrared spectrum, the photosensitive layer being configured toabsorb electromagnetic waves in said third portion of the infraredspectrum.
 2. The image sensor according to claim 1, further comprisingan infrared filter, the color filters being interposed between thephotosensitive layer and the infrared filter, the infrared filter beingconfigured to give way to the electromagnetic waves of the visiblespectrum, to give way to the electromagnetic waves of said first portionof the infrared spectrum, and to block the electromagnetic waves of atleast a second portion of the infrared spectrum between the visiblespectrum and the first portion of the infrared spectrum.
 3. The imagesensor according to claim 2, further comprising an array of lensesinterposed between the photosensitive layer and the infrared filter. 4.The image sensor according to claim 1, wherein, for each pixel of thecolor image to be acquired, the second electrode is common to the first,second, and third sub-pixels.
 5. The image sensor according to claim 1,further comprising, for each first, second, and third sub-pixels, areadout circuit coupled to the second photodiode and to the firstphotodiode.
 6. The image sensor according to claim 5, wherein thereadout circuit is configured to transfer first electric chargesgenerated in the first photodiode to a first electrically-conductivetrack and configured to transfer second charges generated in the secondphotodiode to the first electrically-conductive track or a secondelectrically-conductive track.
 7. The image sensor according to claim 6,wherein the first photodiodes are arranged in rows and in columns andwherein the readout circuits are configured to control the generation ofthe first charges during first time intervals simultaneous for all thefirst photodiodes of the image sensor.
 8. The image sensor according toclaim 6, wherein the second photodiodes are arranged in rows and incolumns and wherein the readout circuits are configured to control thegeneration of the second charges during second time intervalssimultaneous for all the second photodiodes of the image sensor orshifted in time from one row of second photodiodes to another.
 9. Theimage sensor according to claim 6, wherein the readout circuits areconfigured to control a first integration phase for the firstphotodiodes having a first duration and to control a second integrationphase for the second photodiodes having a second duration different fromthe first duration.
 10. The image sensor according to claim 6, whereineach readout circuit comprises at least one first follower-assembled MOStransistor, the second photodiode having a first electrode directlycoupled to the gate of the first MOS transistor and the secondphotodiode having a second electrode coupled to the gate of the firstMOS transistor, or to the gate of a second follower-assembled MOStransistor, via a third MOS transistor.
 11. The image sensor accordingto claim 1, wherein the photosensitive layer is made of organicmaterials and/or contains quantum dots.